Dense wavelength-division multiplexing (DWDM) technology revolutionizes the modern optical communication industry. This is partially in response to the explosive growth of data transmission volume worldwide, for example in the area of the Internet. DWDM communication systems have been developed with the aim of providing high-speed and large-capacity transmission of multi-carrier signals over a single optical fiber. In accordance with DWDM technology, a plurality of concurrent signals, each of which having a different wavelength or frequency, are assembled (multiplexed) at the transmitter end with a wavelength-division multiplexer (Mux) to form a composite signal, which is then transmitted on a single fiber. Each wavelength occupies a signal channel and two adjacent signal channels are separated by a channel spacing, such as 100 GHz at ITU grid. The assembled multi-channel signal is transmitted into a fiber-optic communication network that consists of a set of nodes connected by a link. At the receiver end, the composite multi-channel signal is separated (demultiplexed) with a wavelength-division demultiplexer (DeMux) into their respective wavelength components. Each wavelength signal is then further processed or directed to other networks.
When an optical signal travels down a single-mode fiber, its intensity decays with the distance because the optical fiber has a finite attenuation. For example, the transmission distance is limited by attenuation to about 80 kilometers in a single-mode fiber. In order to keep the signal “useful” to transmit information, the optical signal has to be boosted by amplifiers. Optical amplifiers have been developed and used in strengthening signals without having to convert an optical signal between an electrical and optical form. In fact, it is the advent of C-band erbium-doped fiber amplifiers (EDFAs) that makes DWDM communication networks prevalent. Nowadays, EDFAs can cover both C- and L-band. Semiconductor and Raman optical amplifiers, which greatly extend transmission span, are commercially available.
Optical amplifiers have two key advantages. First, optical amplifiers are transparent and support any bit rates and data formats. Second, optical amplifiers strengthen optical signals in the entire operating wavelength range. For example, an erbium-doped fiber amplifier amplifies all wavelengths in C-band from 1528 nm to 1565 nm.
However, incidents affecting the signal quality such as uneven channel power distribution, wavelength drifts, and optical signal-to-noise ratio (OSNR) may occur in a fiber-optic network where optical amplifiers are used to boost the optical signal traveling along the fiber. Factors contributing to uneven power distribution across individual channels include non-uniform amplification gain of an optical amplifier and the use of optical add/drop multiplexers (OADM), nonlinear process such as stimulated Raman scattering, and so on. This requires optical devices to equalize channel powers or correct power tilt. Dynamic gain equalizers have been utilized to flatten amplified power profiles whereas channel equalizers have been developed to keep individual powers even. The channel wavelengths may drift from their standard values (e.g., ITU grid) due to laser aging, thermal effects, and misalignment of Mux/DeMux devices. Accordingly, wavelength information is important to manage networks. Further, in the use of optical amplifiers in cascade, mode competition, nonlinear processes such as four-wave mixing and stimulated light scattering, will unavoidably increase noise and degrade the signal. It would be desirable to have a spectrometer component that can provide a window at optical layer for monitoring the performance of the network.
Chromatic dispersion causing pulse spreading in time is another factor that limits the transmission distance of an optical signal in DWDM networks. For example, dispersion normal fiber with a dispersion minimum near 1300 nm has its dispersion coefficient of about 17 ps/nm.km at 1550 nm. With chirp free sources, such a chromatic dispersion limits the transmission distance to about 900 kilometers at 2.5 Gbit/s and about 200 kilometers at 5 Gbit/s. This is particularly the case when the data transmission rate becomes higher and higher reaching to about e.g., 10 Gbit/s or 40 Gbit/s. To cope with this issue, dispersion compensators have been developed and utilized to reshape the pulses.
Accordingly, it would be desirable to provide an intelligent, integrated, low-cost, and multi-functional optical signal conditioner at a module level, which provides optical amplification, optical reshaping and retiming.